1. Field of the Invention
The invention relates to fiber optic signal systems, and more particularly to reduction of the distortions produced by modal noise in such systems.
2. Prior Art
Recently published literature describes a hitherto unconsidered source of noise in multi-mode fiber optic systems which use single-mode or other narrow band laser sources. The recently recognized form of interference has been called modal noise, appearing as undesired amplitude modulation of a signal received from the system. The modal noise was described by Epworth, at the Fourth European Conference On Optical Fiber Communication, 1978 in a paper "The Phenomenon of Modal Noise in Analog and Digital Optical Fiber Systems." The phenomenon has more recently been described by Epworth in a paper delivered on Mar. 8, 1979 at a meeting of the Optical Soceity of America in Washington, D.C.
With the exception of a suggestion by Epworth that the best solution for the modal noise problem would be development of a new generation of wide-band emission linear lasers, no disclosures have been made of any practical method or apparatus for adequately reducing such noise.
The present invention provides a means for reduction of such modal noise not previously available in the prior art.